The present invention relates to a gas purification system and method for use in processes such as semiconductor manufacture which are highly sensitive to molecular contamination.
The performance and lifetime of semiconductors are reduced by molecular contaminants. Low temperature epitaxy (LTE) and other silicon processes utilize several gases and are particularly susceptible to impurity problems due to the low temperature. During epitaxial processing, impurities can migrate and become entrapped in the active layer at the device interface. Reliability problems such as low breakdown voltage and increased leakage current occur. These problems all increase as the growth process temperature is reduced.
Gas purifiers have been shown to improve the results achieved by low temperature epitaxial (LTE) processing. However, several drawbacks have restricted their application to only 100% non-blended gas applications. When purifiers are used on mixed gas blends, sometimes referred to as dopant gases, the gas mixture composition changes when initially run through the purifier. Several specialty gases (e.g. Silane, Germane, Diborane, Phosphine, Arsine) are used as mixtures with inert gases or hydrogen in a typical integrated circuit manufacturing process. This gives rise to problems when using purifiers to purify such gas mixtures, particularly if the dopant gas is at a very low percentage of the mixture. For example, when a 1% germane, 99% hydrogen gas blend is run through a purifier, initially all the germane will be removed. Gradually, the purifier will become saturated for germane and the 1% germane will flow through without absorption. However, in order for a purifier to achieve this level, several hours, days, or even weeks of operation may be required before the purifier is fully conditioned. Additionally, if the gas line is allowed to sit stagnant or is pumped down to a vacuum, the purifier must again be conditioned by flowing the gas mixture through the purifier for a significant time period, in order to re-saturate the purifier. Because of this conditioning and reconditioning period each time the process is re-started, purifiers have not, in the past, been applied to gas mixtures, even though such mixtures are known to contain contaminants that adversely affect the process.
It is an object of the present invention to provide a new and improved conditioning system and method for gas purification.
According to one aspect of the present invention, a gas purification system is provided, which comprises an inlet port for connection to a supply of gas to be purified, a process outlet port for connection to a process using the purified gas, a purifier line between the inlet port and the process outlet port, a gas purifier connected in the purifier line between the inlet port and the outlet port, the purifier having an inlet and an outlet, a vent line having a vent outlet port connected to the purifier outlet, a first outlet valve in the vent line for controlling connection of the purifier outlet to the vent outlet port, a second outlet valve in the purifier line for controlling connection of the purifier outlet to the process outlet port, and a mass flow controller in the line for controlling the amount of gas flowing through the purifier.
The system may include a by-pass line connecting the inlet port directly to the process outlet port, by-passing the purifier, an inlet valve in the first connecting line for controlling gas supply to the purifier, and a by pass valve in the by-pass line for controlling connection of the gas directly from the inlet to the process outlet port. A system controller is provided to control operation of the various valves and the mass flow controller between various modes of operation. When the system is not in operation, or in an idle mode, all valves will be closed. When the system is turned on, the purifier will first be connected to the vent outlet by opening the first outlet valve and closing the second outlet valve. At the same time, the mass flow controller is controlled to provide a high flow rate, so that the purifier is conditioned rapidly. Once the purifier is conditioned, the first outlet valve is closed and the second outlet valve is opened, supplying purified gas to the tool or process. The mass flow controller measures the total flow through the purifier during conditioning to determine when conditioning is complete, so that the purifier can immediately be placed back in-line with the outlet port to the process or tool.
In one embodiment of the invention, a gas purification system for gas mixtures is provided, in which several different gases are supplied to a tool in different proportions along separate supply lines, and each supply line is provided with an appropriate gas purifier, with at least some of the gas supply lines being provided with a mass flow controller and a vent outlet as well as a process outlet, with valves for controlling purifier outlet connection to the vent outlet or process outlet, and each gas supply line having a bypass line connecting the gas inlet directly to the process outlet, bypassing the purifier, with a bypass valve in the bypass line for controlling bypass of the purifier. A suitable controller is provided for controlling operation of the mass flow controllers and the various valves in each gas supply line, such that the gas flowing through each purifier is connected to vent until the purifier is conditioned, and is then automatically connected to the process outlet. The controller may be arranged such that unpurified gas is supplied to the process outlet via the by pass line during conditioning. Once the purifier is conditioned, the purifier outlet will again be connected to the process outlet, and the bypass valve will be closed.
The entire gas purification or manifold system for the various gases can be mounted in a single, compact housing with a suitable display unit for indicating process conditions. This makes it quick and easy to incorporate this system in an existing tool or process, simply by connecting the housing between the various gas supply inlets and the tool.
According to another aspect of the present invention, a gas purification method is provided which comprises the steps of:
connecting a gas purifier inlet to a gas supply;
connecting an outlet of the gas purifier to a vent outlet for a predetermined time period sufficient to condition the purifier; and
closing the vent outlet and connecting the gas purifier outlet to a process inlet using the purified gas after the purifier is conditioned.
The gas supply may also be connected through a bypass line to the process inlet while the purifier is being conditioned, with a mass flow controller in line with the purifier controlling the amount of gas flowing through the purifier and the amount of unpurified gas supplied directly to the process inlet. Once the purifier is conditioned, a valve in the bypass line is closed, and the purifier outlet is connected to the process inlet. This arrangement avoids any tool or process downtime as the purifier is conditioned. Additionally, the mass flow controller and valves may be controlled in order to flow a small amount of gas through the purifier to the vent whenever the process or tool is inoperative, avoiding the need to re-condition the purifier after a process shut down, dependent on the duration of the shut down.
The gas purification system and method of this invention permits blends of gases to be conditioned without needing to purge the purifier through the tool, and also allows the purifier to be by-passed to feed the tool directly during purifier conditioning. The purifier can also be reconditioned immediately prior to placing the purifier back online to feed the tool after a shut-down. The mass flow controller allows the purifier to be conditioned much faster, by running a larger amount of gas through the purifier to the vent during conditioning, and then reducing the gas flow to process requirements after conditioning is complete. Also, the total flow through the purifier can be monitored in order to determine when conditioning is complete, and also to determine when a purifier is likely to have reached the end of its expected life.